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Thin anisotropic layer in the mantle wedge beneath northeast Japan

¹ Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan

View larger version (26K): [in this window] [in a new window]   Figure 1. Deformation mechanism map for olivine as function of stress and temperature at pressure, P = 2 GPa, grain size, d, of 1.0 mm, and water content, COH = 1000 ppm H/Si. Thick lines represent transition between diffusion and dislocation mechanisms, and dashed line is transition between climb-controlled power-law creep and glide-controlled Peierls mechanism. Constant strain rate curves of 10–5, 10–10, and 10–15 s–1 are shown as thin gray lines.

View larger version (17K): [in this window] [in a new window]   Figure 2. Strength envelope for lithospheric mantle calculated at a strain rate of 10–14 s–1, water content, COH = 1000 ppm H/Si, and the geotherm of McKenzie et al. (2005). Strength is controlled by frictional sliding at shallow depth and by plastic flow at greater depths.

View larger version (26K): [in this window] [in a new window]   Figure 3. Distribution of anisotropic layer (dark gray region) in mantle wedge beneath northeast Japan, as inferred from dominant deformation mechanisms. Thermal structure is taken from Peacock (2003), and stress contours are calculated from the rate-controlling flow law assuming a strain rate of 10–14 s–1, grain size of 1.0 mm, and water content, COH = 1000 ppm H/Si. Deformation throughout most of mantle wedge occurs via diffusion creep: anisotropic region dominated by dislocation creep is limited to thin layer located above subducting slab and beneath island arc crust.

View larger version (23K): [in this window] [in a new window]   Figure 4. Relation between shear-wave delay time and thickness of anisotropic layer. If we employ strength of anisotropy based on results of experiments on olivine aggregates (Katayama and Karato, 2006), observed delay times of shear waves beneath northeast Japan (0.06–0.26 s; Nakajima and Hasegawa, 2004) can be explained by an anisotropic layer in mantle wedge with thickness of ~5–25 km. AVs—anisotropy for a specific propagation direction; Vs—average velocity of the fast and slow velocities.